Discussions in Genetics



Chapter 1....... Sex, Chromosomes, and Genes.

If you’re new to pigeon genetics, it would be best to first review some of the genetic terms and their definitions listed on the Glossary of Genetic Terms page.    Like most of you, I’m not a trained Biologist nor a Veterinarian.  I'm simply a novice in the sport of pigeons.   What I write about is taken from other genetic writer’s and their works.   I simply try to put it into laymen’s terms and simplify the subject as best I can.  Since most of you are new to this subject I try to write in a style where I often repeat myself.  That is done to reinforce the learning process.  For those more learned types, please be patient and bare with me. 

Okay, with that understood I would like to limit this first chapter to sex, chromosomes and genes.    For starters, that should be more than enough to wet your whistle and get you off to a good start of understanding pigeon genetics.

To begin, let’s discuss the difference between a gene and a chromosome. Some pigeon flyers speak as though they are the same things.  THEY ARE NOT THE SAME.   A chromosome is a thread like, linear strand of DNA and associated proteins.  Another way to describe it is to say that a chromosome is several genes linked together in a string.  In other words, a chromosome is a very small string or chain of genes all combined together in a set fashion. Please refer to   Graphics # 1 & 2   

Each and every gene has it's own unique location point on its assigned chromosome.  This location point is called a Locus.  Genes will always be found at their prescribed locus point, on their assigned chromosome.   Even when mutated they will continue to only be found at their original locus point. The mutation may cause a genetic change in its function but not in its chromosome location or locus.  However, should a mutation occur, you would then have two possibilities for this same locus. These possibilities are referred to as being Alleles.  Some genes have undergone several mutations, resulting in an increase in allele possibilities.  Since only one can exist at a time, only one will be present.  The mutation may have increased the number of gene possibilities within the overall gene pool of the species but not the total number contained in the individual.

Life begins with the fertilization of an egg by a sperm.  This first new complete single cell is called a zygote.  I refer to it as being complete as it contains the entire DNA or building blocks required to develop into a complete being.  Where did this package of DNA come from?  If you need a facts of life lesson I’ll leave it to others, but I will add that the combining of both the male's sperm and the mother's egg forms the Zygote.  Both egg and sperm cells are know as gametes.  Gametes possess only half the total number of chromosomes necessary for new life to begin.  Since chromosomes come in pairs one set of each chromosome pair are contained in the gamete (egg / sperm) cells.  Their selection from the donor parent is said to be at random.  However, its not simply all of one set or all of the other but a mixture of the two.  In fact, there is even a mixing of genes between the two possible chromosomes when making this selection.  In forming this new chromosome, chunks or segments of the gene chains are replicated from each chromosome set and recombined to form new ones.  This allows genes to cross over from one set to the other and results in every individual being uniquely different.   Of course identical twins and or clones are the exceptions. 

Let me pause here and emphasize this one very important point.  In this replication process of a chunk from this one and a chunk from it's matching set member, the total numbers of genes and their actual locus points on the chromosome remains the same.   Its like two different picture puzzles cut from the same mold.  The individual pieces would be interchangeable but when an interchange takes place the picture mosaic changes but the puzzle size and shape remains exactly the same. 

So what happens if this new pair of chromosomes or set contains genes which are not identical?  In other words, are heterozygous having differing alleles present.  If genes are genetic DNA instructions what would be the outcome from these different sets of instructions?  Would they cancel each other out?  Would they have an accumulating effect?   Would one yield to the other?   These are very interesting questions. 

We asume there won't be a canceling effect between the alleles of the same matching loci since we don’t see this happening.  What we do see is an accumulating effect for certain co-dominant genes and cases where the weaker gene yields to the more dominant.  What all this means, is that there are different classifications for gene types.  These are classified as being recessive, dominant, co-dominant or complete dominant.

For example, say the wing shield pattern gene on one chromosome calls for a bar pattern while the other in the set calls for a checker.  We don't expect a bird with one side being a bar while the other is checker, even though such abnormalities do exist as mosaics.  However, mosaics are an entirely different issue and we can save them for another time.  Back to our example of the pattern gene.  Here the gene for checker is found to be dominant and it is the one displayed.  The checker overrides the bar and we see a heterozygous checkered pigeon.  Had the two possibilities both been checkered we would see the exact same phenotype or checkered pigeon; but it would be homozygous or pure for the same genotype.

Okay lets pause for just a moment and go over some of the terms used thus far.  We had genes as being the smallest unit of complete DNA.  Chromosomes as being strings of genes, with each gene having its own locus point.  We learned that genes have their own set of family members, which are said to be alleles.  We also learned that these alleles all share the same locus point and that when a chromosome pair has identical genes at corresponding locus points the chromosomes are said to be homozygous or pure for that single gene factor.  When not pure it is said to be heterozygous.  In the case of heterozygous the more dominant is displayed.  A recessive gene always gives way to it’s more dominate alleles and can only be expressed when the genotype is pure or homozygous for it’s expression.   We found that genotype is the genetic make up while phenotype is what we see.  Heterozygous phenotypes do not display their true genotypes. 

One simple way to remember the difference between heterozygous and homozygous is to remember that hetero is from Greek for “different” while homo means “same”. 

Well that’s a start.  I hope it didn’t leave any of you with your head spinning.  Since I’m an old Air Force retiree let me explain the whole thing using an analogy where an aircraft is representative of a pigeon, and a passenger one of it’s genes, and so on.  In this way I will review some of what we’ve covered and then expand it to include how sex is determined and even get into the three basic pigment colors.

A passenger (gene) has an assigned seat (locus point) in an assigned seating isle (chromosome).   When on board the plane (pigeon), they (the genes) are to be found at their assigned row (chromosome) and in their assigned seat (locus point).  Well genetics is like that.  Any gene found in a pigeon is always found on it’s assigned chromosome and at it’s assigned locus point.  In this analogy, the pigeon is the plane, the chromosomes the seating isles and the seat a particular gene locus point.

Our particular plane has 40 rows on the right side and 40 on the left for a total of 40 pair of rows or 80 rows in total.  Some of these seating rows have more seats than others do.  At the front of the plane each row has only four seats.  As you move towards the middle you find the rows are larger and contain more seats. We notice that one row is very different in that there are no seats on one side of the isle where a storage area exists.  We notice that some planes do not have a storage row and the missing seats are no longer missing.  These planes have more total seats than those with the missing seats where the storage row exists.   If you look close at every plane, you find that each row has the same number of seats as the matching row on the other side of the isle.   The only the storage row with the missing seats are the exception.

Like our plane above, all domestic pigeons have a a total of 40 pairs or 80 chromosomes.  Each chromosome pair will have the same number of genes as it's opposite within the set.  The only exception is the sex chromosome pair known as Z and W.  All male pigeons will have two Z chromosomes and every hen a combination of one Z and one W.  We know that the Z chromosome carries many different gene locus points while a W Chromosome doesn't seem to have any genes.

The W chromosome is sort of like our storage isle with no seats.  A plane with the storage isle would be a hen having a Z and a W; while a plane without the storage isle would be a cock with two Z rows and because of this it would have more total seats.

When the normal or original wild type gene is not present, another in the same family of gene alleles, a mutation, will be.   In other words, the genes are not just strung together at random.  This prevents the possibility of more than one gene of each type existing in the same chromosome.    Example: a bird can not have two genes for leg scales on the same chromosome.  It can however have the same gene on each of the two chromosomes within the set.  Our aircraft can not seat a pair of twins in the same seat.  However, it can seat one in each row opposite each other.  It doesn’t matter if the twins are identical (homozygous pure) or maternal (heterozygous different) they are seated separately but at the same opposite seat / row assignments.

If our pigeon did have more than one copy of the same gene on the same chromosome then something would have to be excluded from the total mix since there wouldn’t be a seat for it.   Remember every bird is a complete package of chromosomes and their assigned genes.   Remove, add, or exclude some chromosomes and we have a different species.   This is why Nature has assigned all three color genes allele to the same locus point.   It insures the one selected is guaranteed a place (locus) on the chromosome and that there is only one allele there.   The same is true for every facet involved, be it pigments color (brown, blue/black and ash-red), wing size, leg scales or whatever.

Every parent donates to their young one chromosome from each of their 40 chromosome set pairs.   In other words, each egg and each sperm will donate the same number of chromosomes or half the required number for that species.  When combined they will have received the same total as their parents and the grand parents before them.   Dr. Willard F Hollander states in his "Origins and Excursions in Pigeon Genetics" that there are at least 70.   Many of these are just tiny dots at 1500x magnification and only around 26 have enough size to appear like a dash and only about 12 are sizeable (over 1/4" at 1500X ).    Since each chromosome is composed of several hundred, perhaps thousands of different gene types, the overall total number of genes is not known at this time.  Please keep in mind that Dr. Hollander wrote this some years back and by the time of his death he believed that number to be at least 40 pairs or 80 total.   

The sex chromosomes are the only exception to the chromosome matching set and size rule.  With these, there are two different types for pigeons, i.e. W and Z.   The W is very small and only found on a hen along with one Z while a cock has two Zs making up his sex chromosome pair and both Z's will be large with a large number of assigned genes.   

As you get into the study of pigeon genetics, you quickly notice some writers will refer to the sex chromosomes as X and Y while others will use the symbols W and Z.   OK you ask  "Which is correct and why?”   That was my question to Dr. Hollander at the

American Pigeon Fanciers' Council meeting in 2000.  He explained that if we rely on the findings of genetic researchers, then we should use the same terms they do.  

Dr. Hollander went on to explain that in some life forms the male of the specie will have two sex chromosomes and will be classified as homogametic while in other species the female is the homogametic member.     Homogametic means one type of gamete for reproduction.    When the female is the homogametic member the symbol X is used for her large sex chromosome or X//X set while Z is used when it is the male that carries the two large sex chromosomes or Z//Z set.

For us humans and other mammals the sex chromosomes are symbolized as X and Y.    X, the larger of the set will have many genes attached while the Y is very small with only a few genes present.    The presence of at least one of the larger sex chromosome regardless if it is classified as an X or a Z is necessary for life.

In humans, it is the female with her two X//X chromosomes that is classified as being the homogametic sex of our species.   In other words, she is only capable of producing gametes with an X chromosome present.    However, as far as the sex chromosomes are concerned it is the male in mammals that produce two different types of gametes i.e. X//Y.   He then is classified as being the heterogametic sex member.

There are some major differences between mammals and some other forms of animal life.    One such difference is when the sex homogametic member is the male and not the female.   Our pigeons along with some other birds, fishes and some forms of insects have the male as being the homogametic sex member.   Therefore he will be symbolized as Z//Z and the female being the heterogametic sex member with a Z//W combination.   

As you can see, this is just the opposite of mammals and is why the symbols were changed from X and Y to Z and W for some life forms.    This change helps clarify which sex is the homogametic member (X//X = female or Z//Z = male) and which is the heterogametic sex member ( X//Y = male or Z//W = female).  The two sex chromosomes in pigeons are therefor correctly symbolized as Z and W and not X and Y.   

As with mammals the larger chromosome of the sex set (in this case the Z) will have many genes whereas the W like the Y is very small and to our knowledge the W contains no genes.    The presence of at least one Z, just like the X, is necessary for life.

Okay, before you get too comfortable with all this, let me blow your mind with some additional facts.   Many forms of insects, such as honeybees, grasshoppers, roaches and true bugs, will have the male of their species with no homologous pairing chromosome.    In other words, he will only have a single X while the female has the standard X//X pair.    In this case the symbol 0 is used to denote the missing chromosome.    We then have X//X = a female just as before and X//0 = the male.  


We even have the opposite of this, with some birds having no W chromosome present and in these cases we again use the 0 to denote its absence.    In such cases we have the symbols Z//Z = male just as before and Z//0 = female.

So all in all, there are four different systems within the animal kingdom that relate to the symbolizing of the sex chromosomes.  All of the systems, which utilize an X, will have the female as being homogametic and all that utilize the Z will have the male as being homogametic.   Any with a missing heterogametic chromosome will utilize the symbol 0 to denote its absence.   

In short there are four separate sets or systems.    The first is the X//Y method, second the Z//W method, third the X//O method and the lastly the Z//O method.  Our birds fall under the Z//W method.    Now you know why the difference exists.   It's not because the chromosomes look like an X or a Y or anything like that.    It’s just a mechanism to aid those that work with these sort things.   It helps them keep the four different conditions straight while dealing with the application of the many genetic principles involved.

This first chapter is titled sex, chromosomes and genes.   So lets wrap it up with a short discussion of basic color and how it relates to what we have covered.

One important genetic factor to remember deals with the locus known as the b locus. This b locus is so named for the mutation from wild type blue/black to brown, thus the name b. The DNA coding at this locus normally results in wild type blue/black color; or for one of its two mutations for brown or ash red. These three gene alleles do not code for the production of color pigments per say; but just the same they do play an important role in determining which type of pigment it will be. For this reason, many writers will refer to this b locus, as the pigment color locus; however, technically that is not correct, even though it does play a role in the pigment color process. Now to the point I am trying to make.   This b locus resides on the sex or Z Chromosome and it is a good example to demonstrate how only one gene allele at a time can reside there.    As stated above, the three basic possibilities or alleles for pigment color are brown, blue/black and ash-red with the blue/black pigment color being the normal or original wild type allele.  The other two, brown and ash-red, are each mutations from the original wild type color of blue/black; and therefore, when present is the gene to be found at this b locus.  In other words, only one gene allele for any locus will reside there.   Since a hen only possesses one Z Chromosome she can only possess one allele for basic color.   It follows that this allele will be from the parent that passed the chromosome DNA where it is located.  For hens, genes located on the sex chromosome can only come from their sire.  For young cocks, the genes that are to be found on his two sex chromosomes will have come from both parents, i.e. 50% from each parent.

With the cock having two Z Chromosomes it naturally follows that he can only pass along what he has.  In this case it’s one of the two Z’s.  Therefore every pigeon sperm (gamete) will contain one Z chromosome.   We know hens on the other hand have both a Z and a W and can pass along either a Z or a W to her egg but not both at the same time.

What happen when she passes a W chromosome in her egg?  Good question.

Answer: it will be combined with the genetic material passed by the sire, which we know is always a Z chromosome.   Remember this is his only possibility.    We end up with a youngster that has both a W and Z chromosome and it will therefore become a hen.    On the other hand if the mother passes along a copy of her Z then the youngster will become a cock since he will also receive a Z type from his sire. Two Z’s equal a cock.  This is why a hen can only receive certain traits or genes from her father and never from her mother.   Basic color being one of these.  In fact all genes found on the hens Z are always from her sire.  However. this is not true for a young cock.  

In a future chapter I will go into the color process a little deeper and show how color and even pattern can under certain conditions tell us if a birds pedigree is truthful or not.

 

Ronald Huntley

   Graphics 1, 2, & 3


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